The present invention relates to reinforced films of water soluble polymers and stable polymeric dispersions which can be dried to produce such films. Specifically, the invention relates to dried films of water soluble polymers reinforced with noncoalesced water insoluble polymer particles where the film comprises a water insoluble component and a water soluble component.
Starch is a water soluble film forming polymer but starch films are not used in many applications because they are brittle, generally low in strength, do not give substantial gloss in paper coatings, and are frequently incompatible when mixed with other polymers. Although the best starch films are thought to be formed by starch polymers having high molecular weights and high concentrations of straight chain polymers, such aqueous solutions of starches have undesirably high viscosities (it being understood in the art that starch is a water soluble polymer). Thus, to prepare strong starch films, very dilute starch solutions are used, which require the removal of very large amounts of water through an expensive drying operation.
One way to reduce the viscosity and increase the solids prior to film formation is to thin the starch by hydrolyzing the starch polymer chains. Unfortunately, such thinning reduces the strength and flexibility of the resulting film. As the starch films are dried, the polymer chains hydrogen bond to one another resulting in a great deal of shrinkage. Shrinkage leads to the creation of stresses in the finished film, which can distort, craze, curl, or crack the film. In addition, hydrogen bonding between starch molecules causes a phenomenon known as "retrogradation" or "setback." In this case, the chains can bond together so strongly that they will not redisperse, even in boiling water. Other carbohydrate and non-carbohydrate film forming water soluble polymers suffer from many of these same problems. Various attempts have been made in the art to improve the properties of starch and other water soluble films but most have provided only marginal improvements in properties. One improvement of interest has been to increase the linear chain fraction of starch, which is amylose. While this has provided films with increased strength, the films have had limited utility because of high cost, difficulty of dispersing the original high amylose starch granules, and increased ease of retrogradation of the linear molecules. To be useful, these film have required addition of plasticizers or humectants. Again, other carbohydrate as well as many non-carbohydrate water soluble film formers have similar difficulties.
Mixtures of starch or other water soluble polymers and latexes have been used as film forming compositions for a long time. However, in these cases, the films which are formed consist of regions of coalesced water insoluble latex polymer particles interspersed with domains rich in the hydrophilic water soluble polymer. It is believed that the formation of a film from latex by coalescence involves a first step of water loss followed by association of the polymer particles ultimately followed by intermixture of polymer chains resulting in loss of the discrete particle structure.
These films from mixtures of water soluble polymers and latexes are thought to consist primarily of a coalesced latex continuous phase, interspersed with regions or domains rich in the water soluble polymer. No effort is made to prevent the latex from coalescing. Indeed, efforts are made to encourage coalescence of the water insoluble polymer particles because it is believed that such coalescence is necessary to form a continuous film having desirable properties including strength.
One area where such materials are used has been in paper coating. In paper coating applications starch is often blended with latex to form the binder system which, when mixed with pigment, forms a basic coating color. Unfortunately, films with coalesced polymer particles used as binders in paper coating compositions tend to shrink. They also have greatly reduced water dispersibility compared to water soluble polymers. Moreover, when latex particles coalesce to form a film, the resulting shrinkage can contribute to mottle-type defects which can be deleterious to gloss. Coatings comprising starch alone as a binder are weak and tend to shrink, resulting in a rough surface having low gloss. Thus, there exists a need in the art for aqueous dispersions having improved film forming properties which can be used as binders in coating color compositions and other end uses.